1. Field of the Invention
The invention relates generally to the field of nuclear magnetic resonance tools. More specifically, the invention relates to nuclear magnetic resonance tools having magnets that are movable so as to control the depth of investigation for the tools.
2. Background Art
Nuclear magnetic resonance (NMR) can be used to determine various characteristics of subsurface formations and/or samples. NMR logging tools can be used downhole to obtain these characteristics, which then can be used to assist in the determination of, for example, the presence, absence, and/or location of hydrocarbons in a given formation or sample.
Conventional NMR logging, well known in the art, generally involves deploying in a wellbore an NMR instrument, which uses magnetic fields to generate and detect various RF signals from nuclei in a formation or sample. Certain example NMR techniques are described in U.S. Pat. No. 6,232,778 assigned to Schlumberger Technology Corp., the entire disclosure of which is hereby incorporated by reference.
More specifically, NMR involves the perturbation of spins systems with a Radio Frequency (RF) field in the presence of a polarizing static magnetic field, Bo. The frequency of operation is defined in terms of the Larmor Frequency (ω—0) where ω—0=γB—0. The Bo field is defined by the magnet geometry and typically is fixed in place. A RF pulse, B1, is then applied to perturb the spin system by applying energy at the resonant condition and orthogonal to Bo. The oscillating signal is then received by the same, or different, coil and the NMR properties of the sample can be calculated.
Logging While Drilling NMR typically uses only one frequency of operation. This frequency corresponds to a depth of investigation inside the formation. Various wireline NMR tools have shown that being able to acquire data at multiple depths enables an accurate measurement of the invasion of drilling fluids into the formation. Additionally, conventional logging-while-drilling (LWD) NMR tools include permanent magnets that are generally mounted in a relatively fixed position that does not allow for movement.
Because of the limitations of conventional LWD NMR systems, it is extremely difficult to acquire data at multiple depths of investigation. Accordingly, there is a need in the art for methods and systems for obtaining NMR measurements that overcome one or more of the deficiencies that exist with conventional methods.